Until now, solid-state lasers (SSL) are widely known as lasers for obtaining high output power, which use optical materials, such as YVO4 and YAG, doped with rare earth elements including neodymium. In recent years, a semiconductor laser excitation solid-state laser using as its excitation light source a semiconductor laser (LD) has become the mainstream in order to achieve reduction in size and high efficiency. Furthermore, in order to obtain high output power, the semiconductor laser for excitation uses in many cases a semiconductor laser array (LD array) in which a plurality of semiconductor lasers is arranged in a direction perpendicular to the optical axis of the semiconductor laser.
The semiconductor laser excitation solid-state laser is not only used for machining applications, but also used as a light source for a display, because visible light can be obtained by halving the wavelength of the solid-state laser through second harmonic generation (SHG) using a nonlinear optical element.
The semiconductor laser excitation solid-state laser module comprises: an LD and a solid-state laser element; a heat-sink for cooling these elements; and a sub-mount for relieving linear expansion stress between the LD and solid-state laser elements and the heat-sink. Moreover, in order to couple laser light output from the LD element with the solid-state laser element and control a transverse mode when the solid-state laser oscillates, a coupling lens being interposed between the LD element and solid-state laser element is often provided. (See, for example, Patent document 1.)
On the other hand, a method has been proposed, in which the solid-state laser element has a planar waveguide structure and thereby a transverse mode in a thickness direction of the waveguide, that is, a vertical transverse mode is controlled, and a transverse mode in a width direction of the waveguide, that is, a horizontal transverse mode is controlled by a thermal lens effect produced inside the solid-state laser element, thereby eliminating the coupling lens, so as to realize a more compact semiconductor laser excitation solid-state laser module. (See, for example, Patent document 2.)
Patent document 1 is an example in which control of the transverse mode of the laser light is achieved by the coupling lens without using the waveguide structure for the solid-state laser element. Patent documents 2 and 3 are examples in which the solid-state laser element is provided with the planar waveguide structure so as to control the vertical transverse mode of the laser light, and in addition the solid-state laser element is disposed on the sub-mount provided with stripe irregularities, whereby thermal distribution is created inside the waveguide with the absorbed laser energy used as a heat source, thereby controlling the horizontal transverse mode of the laser light.